In recent years, the increasing density of microelectronic devices on integrated circuits has lead to a technological bottleneck in the density of metallic signal lines that can be used to interconnect these devices. For example, increased signal-line density has led to difficulties with synchronizing the longest communications links between electronic devices and crosstalk between adjacent signal lines. As a result, rather than transmitting information as electrical signals via signal lines, physicists and engineers are investigating materials and devices that can be used to transmit the same information encoded in electromagnetic radiation (“ER”) through free space or via waveguides. Transmitting information encoded in ER via waveguides has a number of advantages over transmitting electrical signals via signal lines. First, degradation or loss is much less for ER transmitted via waveguides than for electrical signals transmitted via signal lines. Second, waveguides can be fabricated to support a much higher bandwidth than signal lines. For example, a single Cu or Al wire can only transmit a single electrical signal, while a single optical fiber can be configured to transmit about 100 or more differently encoded ER signals.
Advancements in materials science and semiconductor fabrication techniques have made it possible to develop photonic devices that can be integrated with electronic devices, such as CMOS circuits, to form photonic integrated circuits (“PICs”). The term “photonic” refers to devices that can operate with either classically characterized electromagnetic radiation or quantized electromagnetic radiation with frequencies that span the electromagnetic spectrum. PICs are the photonic equivalent of electronic integrated circuits and may be implemented on a wafer of semiconductor material. In order to effectively implement PICs, passive and active photonic devices are needed. Waveguides and attenuators are examples of passive photonic devices that may be used to direct the propagation of ER between microelectronic devices, and photodetectors are examples of active photonic devices that can be used to encode data in ER, detect data-encoded ER, or control the operation of certain microelectronic device components of a PIC. Most photodetectors are p-n or p-i-n junction semiconductor photodiodes. When a pulse of ER having sufficient energy strikes the photodiode, electron-hole pairs are created. The intrinsic electric field of the photodiode then sweeps the electrons and holes in opposite directions through the junction depletion region producing an electric current that can be used to verify the presence of the incident ER pulse or to control the operation of the microelectronic device. For example, a photodiode in electrical communication with a microelectronic device can be used to turn the device on and off by applying corresponding on and off pulses of electromagnetic radiation. However, photodiodes often have a high capacitance, and may require use of an amplifier which may render them impractical for inclusion in a wide variety of microelectronic devices. Physicists and engineers have recognized a need for photonic devices that can be used to enhance the performance and operation of certain microelectronic devices.